The emission of light during the oxidative degradation process of the organics is a part of the reaction course. The first step is the formation of unstable alkyl radicals, which immediately scavenge the oxygen from the atmosphere to form peroxy radicals. These react further and transform into different species in an accelerating degradation cycle (auto-oxidation, left part of figure 1). It is normally attributed to a transition of excited triplet-carbonyl-functions (3R=O*) into their ground state. The spectral range of the light emitted varies according to the type of substances involved. In most cases the chemiluminescence is observed in the short wave region of the visible spectrum from 380 to 450 nm. However, there are well-known exceptions: the relaxation of 1O2 can be detected in the infrared region at ca. 1'200nm.

The required energy (290-340 kJ mol-1) may be supplied by basically three different chemical mechanisms:

a) The combination of two peroxy radicals with concomitant fragmentation in a Russel mechanism is strongly exothermal (460 kJ mol-1). The CL-emitter is an excited “triplet” carbonyl function (right-hand part of figure).

b) The direct homolysis of hydroperoxides followed by a cage reaction leads to an excited carbonyl-function and is combined with the evolution of 315 kJ mol-1.

c) The metathesis of alcoxy or peroxy radicals provides 374 kJ/mol and 323 kJ/mol, respectively. It has been shown, that the CL signal intensity reveals the existence of two kinetic stages during oxidative degradation of organic materials: The first one is corre­lated with the concentration of peroxide groups, the second stage corres­ponds to the oxidation propagation by the hydrogen abstraction responsible for the carbonyl formation.

Simplified scheme of the organic substance's autooxidation (on the left hand) and the so called Russel mechanism (on the right hand): formation of an excited triplet-carbonyl (3R=O*) function by the recombination of 2 peroxy radicals (ROO˙) and its instantaneous relaxation to the ground state under the emission of one photon (chemiluminescence).
Russel mechanism

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